As is well-known, the presence of particles on the surfaces of semiconductor wafers adversely affects the number of acceptable semiconductor devices that can be made from a particular wafer. More stringent requirements for the reduction of particle contamination are continuously being established. The use of megasonic energy to agitate the cleaning fluids employed have helped greatly in particle reduction. Examples of such arrangements are shown in U.S. Pat. Nos. 4,804,007, 4,869,278, 4,998,549 and 5,037,481. U.S. Pat. No. 5,037,481 discloses a transducer assembly wherein a transducer in the form of a thin arcuate segment is coupled to the interior of a quartz tube. Megasonic energy applied to the transducer is transmitted through the quartz tube in a diverging pattern so as to expose both sides of a plurality of wafers positioned in the cleaning fluid in the path of the megasonic energy. In one arrangement, the ends of the quartz tube extend beyond the walls of the container for the wafers and the cleaning fluid such that the electrical connections to the transducer can be easily made through the ends of the tube. In another arrangement, the ends of the tube are closed so that the assembly may be immersed in the fluid.
While the quartz tube assembly provides good cleaning results, there are some shortcomings of the arrangement. Quartz is a desirable material from the standpoint of minimizing contaminants and being compatible with most cleaning solutions, but quartz is incompatible with hydrofluoric acid used as an etching material. Also, while quartz is very hard and hence is a good transmitter of megasonic energy, it is somewhat brittle so that care must be taken in handling and use of quartz tubes so as to minimize the risk of breakage. If a quartz tube should crack, a flammable cleaning fluid could possibly enter the tube and be ignited by an electrical line within the quartz tube. For this reason, the quartz tube array is not recommended for use with flammable fluids.
Applying significant wattage to a megasonic transducer generates considerable heat, and it is desirable that some cleaning solutions be hot. However, the temperature of the transducer must be kept below a predetermined maximum in order to prevent damaging the transducers. With the quartz tube, transducer, coolant, usually in the form of water, is sprayed onto the transducer, or cool nitrogen may be utilized. However, a significant quantity of sprayed coolant is required to enable the transducer to be energized by the desired wattage, and nitrogen gas is a less effective coolant. The transducer electrical connections in a quartz transducer array can be encapsulated so that the tube can be filled with liquid coolant, but megasonic energy could be transmitted into the coolant and wasted. Coolant in gas or spray form does not transmit the energy to a significant degree.
Accordingly, a need exists for a transducer assembly that provides improvements or advantages over an assembly employing a quartz tube.